The field of art to which this invention pertains is dehydrogenation catalysts.
In the catalytic dehydrogenation of alkyl aromatic hydrocarbons to alkenyl aromatic hydrocarbons, e.g., ethyl benzene to styrene, considerable efforts have been expended to develop catalysts which exhibit not only good conversion properties but also high selectivity as well as increased stability.
Typical catalysts used in dehydrogenation of saturated hydrocarbons to unsaturated hydrocarbons, as disclosed in U.S. Pat. No. 2,866,790, are iron oxide catalysts containing a small amount of chromium oxide as a stabilizer and a small amount of potassium compound as promoter. Improved catalysts according to this patent are made from iron oxide (39-47 weight percent), chromium oxide (1-10 weight percent) and potassium carbonate (51-59 weight percent).
Dehydrogenation catalysts having good physical strength are described in U.S. Pat. No. 2,866,791. These catalysts are made from 10 to 60 weight percent potassium fluoride, 0.2 to 20 weight percent chromium oxide and the balance iron oxide.
According to U.S. Pat. No. 3,360,579, a dehydrogenation catalyst, which contains 80-90 weight percent iron oxide, 9-18 weight percent potassium carbonate and 1.5 to 5 weight percent chromium oxide, is prepared by combining yellow iron oxide, chromium oxide, potassium carbonate and water to form a paste, extruding the paste to form pellets, drying the pellets and calcining them at 800.degree.--1000.degree. C. A catalyst made in a similar manner is described in U.S. Pat. No. 3,364,277.
Catalysts having good activity and good selectivity are described in U.S. Pat. No. 3,904,552. These catalysts are made with iron oxide and alkali metal oxides plus molybdenum oxide and cerium oxide.
U.S. Pat. No. 4,404,123 discloses dehydrogenation catalysts which contain iron oxide, potassium oxide, gallium trioxide and 0-5 weight percent chromium oxide.
Other patents which disclose dehydrogenation catalysts based on iron oxide are U.S. Pat. Nos. 4,467,046, 4,749,674, 4,758,543 and 4,804,799.
Dehydrogenation reactions are normally conducted at the highest practical throughput rates to obtain optimum yield. Yield is dependent on conversion and selectivity of the catalyst.
Selectivity of the catalyst is defined as the proportion of the desired product, e.g., styrene, produced to the total amount of feedstock, e.g., ethylbenzene, converted. Activity or conversion is that portion of the feedstock which is converted to the desired product and to by-products.
Improvements in either selectivity or activity but particularly the selectivity of a dehydrogenation catalyst can result in substantially improved operating efficiency.
Low selectivity results in high by-product formation. In the dehydrogenation of ethylbenzene to styrene, the predominant by-products are benzene and toluene. The benzene produced can be recycled for later processing. Toluene cannot be easily recycled and is considered an undesirable by-product. The ratio of benzene to toluene (B/T) in the final product is another criteria to be used in determining the effectiveness of the catalyst.
The activity of dehydrogenation catalysts diminishes with time. Ultimately, the activity of the catalyst is reduced to the point where the catalyst must be regenerated or be replaced. Regeneration and replacement of the catalysts are expensive due to lost production and cost of the catalyst. Any increase in stability of the catalyst, i.e., long term use without diminished activity or selectivity, enhances the economics of the process using the catalyst.
The incorporation of chromium oxide in iron oxide dehydrogenation catalysts, such as those described in U.S. Pat. No. 2,866,790, has been known to improve the stability of the catalysts. However, opposite results are obtained when chromium oxide is added to improved dehydrogenation catalysts, such as those disclosed in U.S. Pat. No. 3,904,552. These catalysts which are described as molybdenum and cerium promoted iron oxide catalysts exhibit improved selectivity and activity in dehydrogenation reactions over iron oxide catalysts which do not contain the promoters. The addition of chromium oxide to such catalysts, rather than improving the stability, actually reduces the time during which the catalysts are effective.